stackctrl: Encode "recursion depth exceeded" message as qstr.

So corresponding exception can be thrown even under tight memory conditions.
diff --git a/py/qstrdefs.h b/py/qstrdefs.h
index 4a34010..32f5b2c 100644
--- a/py/qstrdefs.h
+++ b/py/qstrdefs.h
@@ -385,6 +385,7 @@
 #if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF && (MICROPY_EMERGENCY_EXCEPTION_BUF_SIZE == 0)
 Q(alloc_emergency_exception_buf)
 #endif
+Q(maximum recursion depth exceeded)
 
 Q(<module>)
 Q(<lambda>)